Although acupuncture has been used to relieve pain for thousands of years, recent clinical trials investigating its efficacy on chronic pain have shown contradictory results. This ambiguity may derive from acupuncture's sizeable placebo effects, dosage imprecision as well as highly variable individual response to treatment. The mechanisms underlying these potential confounders remain unknown. In parallel, development in brain imaging provide scientists with new tools for investigating the neurobiological mechanisms underlying acupuncture treatment, and have advanced our understanding of the brain effects of acupuncture stimulation. However, most acupuncture neuroimaging studies to date have been conducted on healthy subjects using a single acupuncture needle in a single treatment session. This gap between the real clinical acupuncture practice and mechanistic research significantly hinders progress in the field and limits the implications of these research findings. In this proposal, we plan to dynamically investigate brain response to verum acupuncture (of two different "doses") and placebo acupuncture, using a paradigm that approximates clinical acupuncture practice across multiple treatment sessions in knee osteoarthritis (OA) patients. This proposal aims to: 1) characterize session-to-session brain responses to verum / sham acupuncture treatment (reliability of response) for OA patients, and 2) investigate how different "doses" of acupuncture influence brain response and acupuncture efficacy (impact of dose). Without such a clear mechanistic understanding of the variability in acupuncture response between different treatment sessions and the variability dependent on acupuncture "dosage," investigation of acupuncture's potential utility for clinical care will be severely limited. We have identified acupuncture treatment for OA as the most critical and urgent study population because acupuncture holds great promise for OA and there is a pressing need to clarify why well-designed acupuncture clinical trials have produced contradictory results. We believe this research takes a critical step forward in applying brain imaging tools to clinical studies of acupuncture. It will help deepen our biological understanding of acupuncture mechanisms and refine future clinical research protocols evaluating the therapy's potential benefits.